The present invention relates to an optical disk apparatus for recording or reproducing information.
The performance of computers has been significantly enhanced in recent years, enabling them to handle various data such as text data, audio data, still image data, and moving image data. To accommodate these types of data, various types of optical disks have been developed and become widely used, meeting the application and required performance of the computers and the forms of data to be handled. For example, optical disks having a diameter of 12 cm are classified into two main types: the CD (Compact Disk) originally developed to distribute music data; and the DVD (Digital Versatile Disk/Digital Video Disk) developed to handle various data in an integrated manner. There are many types of CDs, such as the CD-ROM, CD-R, and CD-RW, and DVDs such as the DVD-ROM, DVD-RAM, DVD-R, and the DVD-RW.
In a reproduction-only CD-ROM or DVD-ROM, information is recorded as a series of pits having convex/concave shapes along a track to be scanned with a light spot. In a recordable optical disk such as a CD-R, CD-RW, DVD-R, and DVD-RW, the track is formed in a groove shape, and a light spot scans the track to record or reproduce information. In the DVD-RAM, the width of each groove and the width of each land between the grooves are substantially equal to each other, utilizing both the groove and the land as tracks in a land/groove structure.
DVDs have a track pitch of 0.74 μm narrowed from the CD track pitch of 1.6 μm, and a surface recording density approximately 7 times as high as that of CDs due to an increase in the linear density. In order to record/reproduce such high-density data, an optical disk apparatus for DVDs has an NA (numerical aperture) of 0.6 and a laser wavelength λ of 660 nm changed from 0.45 and 780 nm, respectively, employed by an optical apparatus for CDs, so as to reduce the size of the light spot.
Each information-recording surface of an optical disk is covered with a transparent substrate to protect the surface against damage and dust. DVDs have a substrate thickness of 0.6 mm, while CDs have a substrate thickness of 1.2 mm. The laser beam is converged by an objective lens through the substrate and irradiated onto the information-recording surface as a small spot. Therefore, if the optical disk is tilted with respect to the objective lens, a coma aberration occurs with the focused beam, asymmetrically distorting the light spot. The coma aberration is proportional to the substrate thickness and the tilt angle of the disk and the third power of the NA (numerical aperture), and inversely proportional to the wavelength. This means that with the same disk tilt angle, DVDs have a coma aberration 40% larger than that of CDs. Furthermore, since DVDs have a track pitch narrower than that of CDs, their information reproduction performance is significantly deteriorated by inclination of the optical disk relative to the objective lens.
In order to reduce the deterioration of reproduction performance resulting from inclination of a disk, various techniques for reducing the influence of the tilt of an optical disk, performed each time the disk is set in place, have been proposed in addition to lens tilt adjustment carried out at the time of production of the apparatus. For example, Japanese Patent Laid-open No. 09-190639 (1997) discloses an optical disk apparatus using a tilt actuator that tilts the entire optical pickup having a tilt sensor attached thereto for measuring the angle of the disk.
On the other hand, the 4.7 GB DVD-RAM disk has a structure in which track groove regions are formed at intervals, and each prepit region is disposed in between to facilitate tilt detection. Japanese Patent Laid-open No. 2000-242951 discloses a technique for detecting a tilt error using the above structure.
Japanese Patent Laid-open No. 08-293126 discloses a method for adjusting the tilt so as to minimize the variation in the pulse width of an information reproduction signal obtained by the optical pickup.
Furthermore, Japanese Patent Laid-open 2001-118274 discloses a method which is used for optical disks having strait grooves and wobbled grooves and which obtains the difference between the amplitudes of wobble signals obtained on an inner circle and an outer circle of a disk, using the difference as a tilt error signal.